The present invention relates generally to the field of stabilization of B-staged dielectric material compositions, In particular, the present invention relates to the stabilization of organo polysilica material compositions.
Organo polysilica materials are well known in art to be useful as precursor materials to organo-silicon containing coatings. Such coatings have found applicability as scratch resistant coatings, insulators (dielectric materials), etch stop layers, hard mask layers, anti-reflective coatings, sound barriers, thermal breaks, insulation, optical coatings and the like. Recently, organic polysilica materials have been of interest as insulating materials.
U.S. Pat. No. 4,349,609 (Takeda et al.) discloses organo polysilica compositions particularly suitable for use as dielectric materials in the manufacture of integrated circuits. Much effort has been devoted to lowering the dielectric constant of such materials by incorporating voids into films of organic polysilica materials. For example, see U.S. Pat. No. 5,895,263 (Carter et al.) and U.S. Pat. No. 6,271,273 B1 (You et al.) which disclose processes for manufacturing an integrated circuit device containing a porous organo polysilica dielectric layer.
The organo polysilica material is typically applied to a substrate in a B-staged form in a solution. Such B-staged material includes monomeric, oligomeric or even polymeric material that is further cured, such as by condensation, to form higher molecular weight materials, such as cured films or coatings. Organic solvents are typically used to prepare such coating solutions of the B-staged organo polysilica material.
The preparation of B-staged organo polysilica material is well-known. Typically, an alkyltrialkoxy silane is reacted with water in the presence of an acid catalyst to form a siloxane partial condensation product. For example, see U.S. Pat. No. 3,389,114 (Burzynski et al.) which discloses the preparation of methyl silsesquioxane by reacting methyltriethoxysilane with water in the presence of up to 700 ppm of hydrochloric acid as a catalyst. Burzynski further found that harder alkyl polysiloxanes following cure can be attained by using greater than 700 ppm of formic acid, see U.S. Pat. No. 4,223,121. A variety of other organic acids are known to catalyze the condensation of alkyl or phenyl trialkoxy or trihalo silanes. Such acids are typically used to buffer the pH of the reaction mixture in order to control the hydrolysis rate. These acids include propionic, butyric, citric, benzoic, acetic, oxalic, chloroacetic, glutaric, glycolic, maleic, malonic, dimethylmalonic, and para-toluenesulfonic. See, for example, U.S. Pat. No. 4,324,712 (Vaughn) and International Patent Application WO 01/41541 (Gasworth et al.).
B-Staged organo polysilica resins are prone to self-condensation, which increases the molecular weight of the resins and thereby affecting other properties of the resins. Most notably, change occurs in coating solutions where the viscosity, coating thickness and uniformity are significantly impacted. Also, the increase in molecular weight results in formation of gel particles which lead to coating defects. This is especially problematic in applications where the optical quality of the coating is important. For example, in optical wave-guides gel particles can lead to regions in the device that cause light to scatter. In scratch-resistant coatings such as on eye glass lenses, gel particles cause aberrations that are unattractive and interfere with the wearer's vision. The increase in organo polysilica resin molecular weight over time and the formation of gel particles leads to a short shelf life for many products that contain organo polysilica resins.
Conventionally, solutions of B-staged organo polysilica resins are stored cold in order to decrease such self-condensation reactions and increase their shelf life. The solutions must be shipped using refrigerated transportation and/or containers. Such cold storage requirements greatly adds to the cost of handling these materials. Thus, there is a need for improved shelf life of B-staged organo polysilica resins.
U.S. Pat. No. 5,993,532 (Broderick et al.) discloses stable hydrogen silsesquioxane compositions containing 100 parts by weight solvent, 0.1 to 100 parts by weight hydrogen silsesquioxane and 0.002 to 4 parts by weight acid. A variety of organic and inorganic acids are disclosed. There is no mention in this patent of organo polysilica resins.
U.S. Pat. No. 6,235,101 B1 (Kurosawa et al.) discloses a composition including a hydrozylate and/or partial condensate of a compound of the formula R1nSi(OR2)4-n, where R1 and R2 are (C1-C5) alkyl or (C6-C20) aryl and n is 1 or 2; a metal chelate compound; an organic solvent having a boiling point of 110° to 180° C.; and a β-diketone. These compositions may also contain a tetrafunctional alkoxysilane, such as tetramethoxysilane or tetraethoxysilane. These tetrafunctional alkoxysilanes are disclosed to provide a degree of storage stability to the compositions. It is also disclosed that certain carboxylic acids can be added to these compositions. However, neither the function of such carboxylic acids nor the amounts of such acids nor the pKa values of such acids are disclosed.
U.S. Pat. No. 2,986,548 (McLoughlin) discloses organopolysiloxane resin compositions containing an alkaline condensation catalyst, such as a quaternary ammonium compound, that have increased shelf life by the addition of a halogenated acid. This patent teaches that such halogenated acids are required to prevent premature curing of the resin by the condensation catalyst. Nothing in this patent discloses the stabilization of organopolysiloxane resins in the absence of such alkaline condensation catalyst.